Engines are usually designed with the ability to deliver a peak output, although most engine operation is performed well below this peak value. As such, it can be beneficial to operate with some cylinders inducting air without fuel injection, and other combusting air and injected fuel, as described in U.S. Pat. No. 6,568,177. By disabling some of the cylinders on the engine when desired output, such as torque, is relatively low, the engine can be operated at a higher manifold pressure to supply the needed airflow to the operating cylinders, thus reducing engine pumping losses and making the engine more efficient. In one example, the system can also use lean combustion and a NOx trap to avoid NOx emissions, and further improver efficiency.
In one approach, cylinder deactivation of all of the engine cylinders is enabled to improve vehicle fuel economy. However, since engine speed may drop when the cylinders are deactivated, a lower threshold engine speed is used, for example 1500 RPM, so that when deactivation occurs, even during a worst case scenario, engine speed will not drop below a minimum allowed engine speed, such as 1000 RPM. This minimum allowed speed is selected to reduce engine stalls that might otherwise occur if the driver takes such action as aggressive brake application.
However, the inventors here have recognized a disadvantage with such an approach. In particular, since the system is calibrated for worst case conditions there are many opportunities for enabling, or continuing, cylinder cut operation, that may be lost. This can result in lost fuel economy.